1. Field of the Invention
The present invention relates generally to radio positioning and communication systems, and is particularly concerned with a radio position determination and message transfer system in which artificial satellites at known locations are employed as relay stations for the ranging and message signals.
2. Description of the Prior Art
The present invention constitutes an improvement or modification of the satellite-based position determining system described in U.S. Pat. No. 4,359,733, issued to G. K. O'Neill on Nov. 16, 1982. In that system, the user vehicles are equipped with transponders for transmitting a uniquely coded beacon or reply signal in response to a received general interrogation signal. Three repeater-carrying satellites are provided at spaced orbital locations above the earth for receiving and retransmitting the reply signals produced by the vehicle transponders. A ground station periodically transmits the general interrogation signal, and also receives and processes the reply signals retransmitted by the three satellites in order to determine the instantaneous positions of the user vehicles.
In order to avoid signal overlap and equipment saturation at the ground station, each vehicle transponder in U.S. Pat. No. 4,359,733 includes means responsive to the general interrogation signal for inhibiting the response of the transponder to subsequent general interrogation signals for a predetermined time interval following the response of the transponder to a first general interrogation signal. This avoids the need for discrete addressing of individual transponders, time-slotted polling, multiple frequencies, and the various other complex techniques which had previously been considered necessary to reduce signal overlap at the receiving station. In addition, the possibility of varying the inhibit interval allows the effective response rate to be modified for different classes of users, or for the same user during different periods of need, without changing the actual interrogation rate at the ground station.
The system described in U.S. Pat. No. 4,359,733 determines user position from the arrival times of the reply signals received from the three satellites, measured against the known transmission time of the interrogation signal from the ground station. The three time differences supply the necessary unknowns for a set of three equations, which can be solved simultaneously to yield the three-dimensional position of the user. This is particularly useful when the user vehicles comprise aircraft, since the altitude of a given aircraft can be determined directly along with its latitude and longitude, without reliance on the aircraft altimeter. By contrast, ground-based radar systems are incapable of determining aircraft height except very crudely, and must rely on aircraft-carried encoding altimeters for precise altitude information.
Although a system for three satellites is desirable from the standpoint of providing complete three-dimensional position information, the significant costs involved in placing satellites into orbit and maintaining the satellites make it attractive to consider whether a lesser number of satellites could be employed. A system of two satellites, for example, saves the cost of one satellite and can also serve as a functioning subset of a three-satellite system in case of a failure of one of the satellites. Although position determining systems relying on two satellites have been proposed in the prior art, these systems have generally depended on the use of vehicle-carried equipment for providing one of the position coordinates. In the case of aircraft, for example, the height coordinate can be provided by an altimeter and combined with the satellite-derived position information to obtain a completed position fix. Unfortunately, this method of calculating position is highly dependent on altimeter accuracy, which can be adversely affected by barometric fluctuations, improper adjustment, and other factors.
In the case of surface-based users such as automobiles, trains, ships and pedestrians, the situation is simplified somewhat since it is not necessary to calculate an altitude coordinate. For these classes of users, a system of two satellites can produce an approximate two-dimensional position fix (i.e., latitude and longitude) if one proceeds from the assumption that the earth is perfectly spherical, or, more accurately, ellipsoidal. Although this assumption is useful when rough or approximate position fixes are desired, it is not appropriate for precise position measurements since local terrain features are not taken into account. Differences in local terrain height on the earth's surface can amount to thousands of feet, and these differences will introduce substantially errors into the horizontal position calculation for surface-based users.